A single-axis actuator, in which a ball screw mechanism and a linear guide mechanism are combined together, has a guide rail having a letter U shaped cross section perpendicular to a longitudinal direction of the actuator, a slider arranged in a letter U shaped depression of the guide rail, and plural rolling elements allocated between the guide rail and the slider. In the slider, a nut is formed in parallel to the guide rail, so that the plural balls are allocated in a raceway formed by a spiral groove of the nut and a spiral groove of a threaded shaft penetrating through the nut.
The guide rail has a rolling face implementing a rolling passage for the rolling elements on each of the inner faces opposing each side faces of the slider. The slider has a rolling face opposing the rolling face of the guide rail so as to implement a rolling passage, a returning passage for the rolling elements, and a direction changing passage for communicating the returning passage and the rolling passage. The rolling elements are allocated in a circulating passage implemented by the rolling passage, the returning passage, and the direction changing passage. The single-axis actuator further includes a circulating part for returning the balls from the endpoint to the start point of the raceway.
Then, the single-axis actuator is configured so that the rotational force of the threaded shaft is transmitted through the balls to the nut by rotation of the threaded shaft, the rolling elements circulate in the circulating passage while rolling in the rolling passage in a loaded state, and the slider moves along the guide rail.
As a conventional example of the above single-axis actuator, PTL 1 and PTL 2 disclose the single-axis actuators into which rollers are incorporated as the rolling elements, respectively. By using the rollers as the rolling elements, it is possible to increase the load capacity and rigidity while ensuring the lightweight and compact size, as compared to the case of using the balls.
In the single-axis actuator disclosed in PTL 1, a female screw part of the bearing block (i.e., the nut of the slider) is formed by insert molding.
In the single-axis actuator disclosed in PTL 2, in order to ensure the light weight and compact size at a low cost, the DF contact structure (i.e., the structure in which the virtual intersection point of the load-effecting lines of rows of rollers rolling in a double-row rolling passage formed on each of inner faces is arranged on the inner side of the rolling passage in the width direction of the actuator) is employed and the nut is directly formed in the slider.
In a conventional single-axis actuator, a circulating tube is used for implementing the ball returning passage for returning the balls from the end point to the start point of the raceway. That is, the slider is configured to have through holes penetrating perpendicularly to a longitudinal direction of the threaded shaft. Leg parts of the circulating tube are inserted into the through holes, respectively, so as to secure the circulating tube onto either the top face or the bottom face of the slider by an attachment bracket. Besides, in the conventional single-axis actuator, the area of the outer circumferential circle of the threaded shaft is less than 20% of the area of the cross section perpendicular to the longitudinal direction of the threaded shaft of the slider.
PTL 3 discloses that the ball returning passage of the ball screw apparatus is configured to encompass the through hole penetrating through the nut in the longitudinal direction of the threaded shaft, and circulating parts (i.e., circulating pieces) having the direction changing passage connected to the through hole and disposed at both ends in the longitudinal direction of the threaded shaft of the nut. In addition, PTL 3 also discloses that the circulating part has a retaining projection for preventing from being dropped out in the longitudinal direction of the threaded shaft (i.e., a projection at a part to be contact with the radial inner face of the nut), and a depression into which the retaining projection is fit is provided on the radial inner face of the nut.
Further, in the case of the ball screw apparatus, the structure with the use of a letter C shaped retaining ring can be considered to be a retaining structure for preventing the circulating parts disposed at both ends in the longitudinal direction of the threaded shaft of the nut from dropping out in the longitudinal direction of the threaded shaft.